The invention relates to a process for preparing 1,6-hexanediol, in which a hexanediol having a proportion by weight of nitrogen of less than 5 ppm is obtained, 1,6-hexane-diol having a proportion by weight of nitrogen of less than 5 ppm and also the use of this 1,6-hexanediol for preparing polymers.
There is a great demand for 1,6-hexanediol which has no amines in amounts which have a catalytic effect in the preparation of polyurethanes, since these catalytic amounts of amines lead to considerable amounts of by-products which hinder the reaction to form the polyurethane.
DE 10112117 A1 describes a process for removing nitrogen-comprising compounds using acidic and/or basic ion exchangers. The proportion by weight of nitrogen is determined here by means of a CPR (controlled polymerization rate). This purification process has the disadvantage that the use of acidic and/or basic ion exchangers leads to increased costs since the ion exchangers themselves represent costs and their use means an increased use of solvents since ion exchangers are only finitely useable and have to be regenerated every now and again. In addition, to avoid losses of product, rinsing of the ion exchanger is necessary and leads to increased use of solvents or regeneration media. Since 1,6-hexanediol is solid under normal conditions, the feed to the ion exchanger has to be additionally heated for a reaction over the ion exchanger to be possible at all. Thus, the process for purifying polyalcohols in DE 101112117 A1 has considerable disadvantages. Furthermore, DE 101112117 A1 does not describe the removal of nitrogen-comprising compounds from 1,6-hexanediol.
The preparation of 1,6-hexanediol starts out from the appropriate cyclo-C6-alkanes, alcohols, ketones and/or mixtures of these compounds, and these are either oxidized in the presence of nitric acid and/or subjected to oxidation with subsequent water extraction of the organic stream.
In the case of 1,6-hexanediol, streams comprising adipic acid are produced in this way from, for example, cyclohexanol and/or cyclohexanone by oxidation using nitric acid. For the present purposes, streams comprising adipic acid are streams which can comprise adipic acid itself or else adipic acid in the form of its esters. In the oxidation, it is possible to use both the adipic acid obtained by oxidation and the mixture remaining after the adipic acid has largely been separated off, which mixture comprises adipic acid, glutaric acid and succinic acid.
Furthermore, other sources of adipic acid or streams comprising adipic acid are in principle streams which can be mixed with the abovementioned streams, for example streams obtained by oxidation of cyclohexane to cyclohexanol/cyclohexanone mixtures and subsequent water extraction of the organic stream.
The abovementioned streams usually comprise impurities which in the case of the oxidation of cyclohexanol/cyclohexanone are formed by oxidation using nitric acid and comprise nitrogen. Nitrogen components are also present as undesirable secondary components in the water extracts after the oxidation of cyclohexane by means of air.
These nitrogen compounds, which can be present, for example, as nitro group, amides or ammonium ion, are able to form amines during the hydrogenation of streams comprising adipic acid, which can also be esterified. For example, nitro compounds can be hydrogenated directly to amines and/or amides. Ammonium ions can aminate alcohols formed during the hydrogenation.
Amines are basic components and as such are undesirable in 1,6-hexanediol since they have properties which are undesirable in the uses of 1,6-hexanediol. Thus, these amines can, for example, have a catalytic action in the preparation of polyurethanes, so that process control for preparing a product having precisely defined properties is difficult if not impossible. It can happen that entire production batches have to be disposed of. This also applies in principle in the preparation of polyesters or polyester alcohols which are then again reacted further with isocyanates to form urethanes.
One possible way of establishing whether undesirable N-comprising compounds are present in the form of basically acting amines in 1,6-hexanediol is to determine the CPR (controlled polymerization rate). The content of basically acting amine is accordingly coupled with the CPR and can, as explained for the example of 1,6-hexanediol, be determined as follows:                30 g of 1,6-hexanediol are dissolved in 100 ml of a solution of potassium hydroxide in methanol (0.001 mol/l) and stirred for 15 minutes. This solution is, for example, titrated potentiometrically with 0.01N hydrochloric acid to the end point using a Titroprocessor 682™ from Metrohm, Herison, Switzerland. The Titroprocessor 682 is equipped with two pH electrodes, viz. a glass electrode (3 M KCl, Metrohm 6.0133.100) and an Ag/AgCl/LiCl electrode (alcohol, Metrohm 6.0726.100). The procedure is repeated using a comparative solution comprising 100 ml of a solution of potassium hydroxide in methanol (0.001 mol/l) to determine the blank.        
The CPR is determined from the two results of potentiometric titration as follows:CPR=10×(V1−V2), where                V1 is the consumption of 0.01N hydrochloric acid in the case of the polyalcohol sample,        V2 is the consumption in the comparison (blank) and        10 corresponds to the calculation factor in accordance with JIS (Japan Industrial Standard) K 1557-1970.        
For example, at a CPR of 10, i.e. a net hydrochloric acid consumption of 1 g of 0.01 molar HCl, about 5 ppm of N is present in the 1,6-hexanediol. Such a CPR of 10, i.e. an N content of 5 ppm, is already an undesirably high level and can cause considerable secondary reactions in subsequent polyurethane reactions.
It is therefore an object of the present invention to provide a process which makes it possible to prepare 1,6-hexanediol having a CPR of less than 10, without an additional outlay and costs associated with additional solvents and/or acidic and/or basic ion exchangers having to be incurred.